Normally, the resolution of a synthetic aperture radar (referred to as SAR below) in the azimuth (abbreviated to Az below) direction, i.e., the moving or traveling direction of a movable platform on which the synthetic aperture radar is installed is about a half of the antenna size in the Az direction. Accordingly, it is necessary to reduce the antenna size in the Az direction in order to improve the resolution in the Az direction. This accompanies with increase in the Doppler bandwidth as shown in FIG. 8, thereby requiring for setting the pulse repetition frequency (referred to as PRF below) high in order to satisfy the sampling theory. On the other hand, if the PRF becomes higher, observation swathwidth in the range (abbreviated to Rg below) direction is restricted. This means that higher resolution in the Az direction and wider observation swathwidth in the Rg direction are trade off. Now, reference is made to FIG. 8 for describing problems encountering when reproducing SAR images in a normal SAR. When the antenna size is reduced in order to provide a wider transmission beam, the Doppler bandwidth (i.e., synthetic aperture bandwidth) becomes wider, thereby requiring for setting the PRF high.
A spotlight SAR illustrated in FIG. 9 has been proposed as conventional means to overcome the aforementioned problem. The spotlight SAR is designed to expand the synthetic aperture bandwidth and improve the resolution in the Az direction by scanning in time in the moving direction and the opposite direction thereto. FIG. 9 describes how to reproduce SAR images by the spotlight SAR. By scanning the transmission beam of a normal bandwidth in time in the moving direction, the synthetic aperture bandwidth of a receiving beam is expanded without expanding the Doppler bandwidth, thereby enabling to produce high Az resolution images. However, the spotlight SAR has the following problems: Firstly, the beam must be scanned in time in the Az direction. Moreover, it is unable to make a band observation (or stripmap observation) that is possible in the normal synthetic aperture radar.
An alternative method is to employ a dual receive antenna (abbreviated to DRA below) mode in a TerraSAR-X as shown in FIGS. 10 and 11. The receiving antenna is electrically divided into two in the Az direction and the PRF is set so that receiving positions (phase centers) of the two antennae are alternatively arranged. By synthesizing or combining the data that are received by the both receiving antennae, the PRF is virtually doubled (this is a technique similar to the one disclosed in J. Mittermayer and H. Runge, “Conceptual studies for exploiting the TerraSAR-X dual receiver antenna” (non-patent document 1). In this approach, it is made possible to have a wide Doppler bandwidth, i.e., a high resolution in the Az direction while maintaining the actual PRF low. FIG. 10 shows how to reproduce SAR images in the DRA mode of the TerraSAR-X. It uses a wide bandwidth transmission beam and synthesizes both receiving data of the two receiving antennae with adjusted phase center of the beams. As a result, the PRF is virtually doubled while maintaining the actual PRF low, thereby expanding the synthetic aperture bandwidth of the receiving beam and producing images of high resolution in the Az direction. FIG. 11 shows the phase relationship between the transmission and receiving beams in the DRA mode of the TerraSAR-X, wherein phase centers of the data received by the two receiving antennae are alternately located in time along the moving direction.
However, in this method of using the DRA mode, it is necessary to set the PRF so that the phase positions (phase centers of transmission and receiving antennae) of the data received by the receiver 1 and the receiver 2 are alternately disposed, thereby making it impossible to freely set the PRF.
Similarly, in Japanese patent publication entitled “Side Monitoring SAR System” (Japanese Patent Publication no. 2004-523760, patent document 1) discloses a technique of achieving both high resolution and wide band observation by integrating a transmission by one small transmission antenna and receiving by a large number of small receiving antennae. In case of the patent document 1, receiving data of a plurality of receiving antennae are synthesized to achieve high resolution similarly to the case of the DRA mode of the TerraSAR-X as described hereinabove, thereby expanding the virtual PRF and achieving a wide Doppler bandwidth, while maintaining the actual PRF low.
However, it is also necessary in the method as disclosed in the patent document 1 to alternately arrange the phase position (phase centers of the transmission and receiving antennae) of the data received by the respective receivers similar to the aforementioned case in the DRA mode of the TerraSAR-X, thereby making it impossible to freely set the PRF.
As apparent from the foregoing descriptions, it was difficult in the conventional SAR to achieve high resolution in the Az direction under the condition of wide observation swathwidth in the Rg direction, the stripmap observation and free PRF setting.